Metallurgical processes of primary iron and steelmaking belong to the most energy intensive of all production processes in the industry as a whole. Therefore, their overall energy balance was always of great interest to metallurgists. Better understanding of limitations of non-renewable energy resources and eventually energy cost crisis in 1970's have initiated intensive activity for lowering of energy consumption also from the side of energy producers and suppliers. Furthermore, ecological considerations and the vital necessity of environmental protection are becoming deciding factors to control all branches of the industry. Since metallurgical processes of iron and steelmaking are leading also in high emissions of air polluting toxic gases, as well of production of solid hazardous wastes, it is logical that they are more and more in the spotlight of attention of public and primarily of government authorities responsible for clean environment. For the above reasons there is world wide effort to the improve energy balance of the metallurgical processes by improving energy efficiency as much as possible. In some measure this has been achieved by replacing one type of fuel or energy with another, more suitable for the specific process or its particular stage, with highest respect and consideration for ecology, economy and availability. The first priority in reducing energy consumption in the majority of smelting and melting metallurgical processes is the highest possible utilisation of the so far unused process system energy losses, such as sensible and chemical heat of waste exhaust gases. By returning part of this energy, by the most direct route possible, into the metallurgical process which produced the waste exhaust gases, initial energy requirements will be reduced, resulting in overall higher energy efficiency of the process. In compliance, most sincere and well thought efforts to utilise waste energy contained in the off-gases, in as possible direct way, lead to designs which incorporate energy recuperating devices for scrap preheating into a current electric arc furnace structure. So far, in contrast with expectations, these state of the art amalgamated electric arc furnace aggregates, of complicated design, are reaching only some of all anticipated performance results. High initial and installation costs, malfunction of mechanisms, extensive maintenance, pollution and safety problems culminating in dangerous explosions are evident reasons for raising questions of their suitability in general. With current and future environment protection rules, these question are becoming increasingly pertinent, since toxic emissions from current state of art scrap preheating devices are not meeting all stipulations and criteria of the valid or proposed regulations for permissible levels of toxic substances emitted into atmosphere.
On the other hand, it was well known, that the most promising and efficient method for indirect energy saving, especially for electric arc furnace steelmaking from scrap, is the high temperature preheating of a metallic charge before charging into the furnace, in a separate heating device, better known as "pre-charge scrap preheating". Be that as it may, because of the lack of a fully developed design of this type of scrap preheating equipment for electric arc furnace steelmaking, it was only sporadically used in an underdeveloped "scrap in bucket preheating" configuration.
Alongside rising energy and ecological concerns there is ever present endeavour to intensify any and all phases of the electric arc furnace steelmaking process, above all increasing productivity and reducing operating costs. For example, further increases of electric power input via optimally increased secondary voltage complimented with adequate foamy slag practice; instantaneous recuperation of chemical energy via post-combustion of combustible gases directly in the furnace vessel before they are exhausted; addition of oxy-fuel burners to the furnace vessel for intensification and acceleration of rapid scrap melting; preheating of the ferrous scrap charge prior to charging into the furnace by using sensible and chemical heat energy contained in the off-gases plus oxy-fuel burners; and finally, introduction of supersonic gaseous oxygen lances for intensification of decarburization and foaming up of slag.
Of the aforementioned process intensification methods, the three methods for increasing temperature of scrap by other means then via electric arc, are: instantaneous recuperation of chemical energy in the vessel via post-combustion; and addition of oxy-fuel burners to the furnace vessel and preheating of the ferrous scrap charge prior to its charging into the furnace vessel.
The intent of the first method is instantaneous recuperation of chemical energy directly in the furnace vessel, by combusting via gaseous oxygen the combustible components of off-gases developed by the process of scrap melting, before being exhausted. This method is being exploited with variable success in open-hearth furnaces, basic oxygen furnaces and energy optimising furnaces. Preheating of oxygen enriched air by sensible heat of the exhausted off-gases from the vessel are used instead of gaseous oxygen as a variant of this method. Nevertheless, success of this method, applied to electric arc furnaces is showing only limited productivity improvement and electric energy saving, primarily when used during the stage of melting scrap simultaneously with electric arc. Actual energy saving is the result of in-situ scrap preheating. Use of post-combustion in electric arc furnaces with already molten scrap is in reality significantly curbed by unsatisfactory heat transfer efficiency from post-combustion gases into the bath, covered by deep layer of thermally insulating and foamy slag. Combustion or so called post-combustion of combustible components of the off-gases emerging from the foamy slag increases the temperature and consequently volume of the off-gases in the free space above the slag. Successive, proportionally increased pressure of the off-gases in the furnace vessel is therefore abating aspiration of the cold ambient air into the furnace vessel. Hence, while keeping the necessary internal temperature of the furnace vessel the same, electric energy requirement for heating of the cold air is eliminated, ultimately resulting in its saving. It should be noted, that in comparison with the post-combustion, requiring additional oxygen at added cost, an equal or higher energy saving, at no cost, is achieved by adequately sealing of the furnace vessel and on that account preventing intake of the cold air. Moreover, a consequent and exceptional benefit of adequate sealing the electric arc furnace is in the drastically reduced quantity of hot off-gases to be exhausted from the furnace. In the case of a 110 tonnes furnace, for example, the quantity of the gases to be handled was reduced more than 50% (from 90,000 Nm.sup.3 /h to 40,000 Nm.sup.3 /h), allowing the stopping of one of the exhaust fans.
The purpose of the second method, for increasing the temperature of the scrap by other means than via electric arc is, intensification and acceleration of rapid scrap melting by addition of oxy-fuel burners in the furnace vessel. Although positive results were obtained from introduction of oxy-fuel burners for faster melting of the scrap in the region of the slag door tunnel of the electric arc furnace vessel over 30 years ago, they have not been used to a great extent until Ultra High Power furnaces with watercooled panels have been built. Beneficial performance of short flame oxy-fuel burners located in the vessel walls in the "cold" zones between electrodes have caused shortening of the time for melting of all scrap in the furnace. These positive results started a fashionable avalanche of burner additions to the vessels of electric arc furnaces. In last few years numerous types of oxy-fuel burner designs with ever increasing capacities have been made available for electric arc furnaces. Currently, the thermal power input of these burners represents in some cases more than 50% of initial electric energy power input. Although this, low cost, addition of overall power input shortens the tap-to-tap times, with the desired productivity increase and induces some to other operating and economic benefits, many other serious disadvantages are overlooked and suppressed. In general, some of the major disadvantages are: higher oxidation of the scrap, larger volume of off-gases, substantially lowered heat transfer efficiency if burners are operated simultaneously with electric arc power input, especially if the burners are operating all the time during the heat. Practical operating results have proved that highest energy efficiency is achieved when the heat is started with burners only, which are substituted with electric arcs only after the charge has reached temperature of about 800.degree. C. This two-stage operating practice resulted in 15-20% electric energy saving and 10-15% saving of fossil fuel and oxygen. However, because of sequential application of both types of thermal energy the tap-to-tap time has increased by 10-12%. Economically, the cost of installation of oxy-fuel burners to the existing furnace vessel is relatively low, yet in almost all cases it resulted in substantial costs for rebuilding and enlarging of the entire exhaust system. From a broad ecological view point, operation of such electric arc furnaces with excessive use of oxy-fuel burners and lowered energy efficiency, produce a disproportionately higher volume of hazardous components contained in off-gases, and such processes are becoming categorically unacceptable.
The objective of the third method, in increasing the temperature of the scrap by other means then via electric arc is, preheating of the ferrous scrap charge prior to charging into the furnace by efficiently using sensible and chemical heat energy contained in the off-gases plus use of oxy-fuel burners, if necessary for ecological reasons and concerns.
From its introduction, scrap preheating went through several development stages: batch preheating in the charging bucket with hot waste gases from the furnace or with air- and oxy-fuel burners; continuous preheating via inclined rotating kiln or horizontal vibrating conveyor using a combination of hot waste gases from the furnace and air- and oxy-fuel burners; continuous vertical preheating mechanism with controlled scrap descent, being an integral part of the furnace and using hot waste gases from the furnace in counter current flow; and as well "in situ" preheating of the scrap already charged into the furnace at the beginning of the heat simultaneously with electric arc via a variety of different designs of air- and oxy-fuel burners. There are several other unique scrap preheating mechanisms being combinations of the above discussed systems and operating with more or less success.
Currently scrap preheating is gaining long time overdue recognition. By recognising its great potential, it is now considered that it will be the next production process milestone for electric arc furnace steelmaking mainly from scrap, with respect to electric energy saving, reduction of electrode consumption, productivity increase by shortening the tap-to-tap time and the very important benefit of reduction of environment pollution in general.
From experience with the process of steelmaking in an electric arc furnace predominantly from recycled mixture of ferrous charge--cold steel scrap, it could be concluded, that adequate preheating of the scrap prior to charging into the furnace for rapid efficient melting is the most suitable method.
In the recent past, several types of equipment and processes for preheating scrap have been introduced and made available to the electric arc furnace steelmaking industry, generally in accordance with the following U.S. patents:
U.S. Pat. No. 4,543,124 (24.09.1985) describes an "Apparatus for continuous steelmaking", known in the industry as "Consteel Process". The process uses the furnace off-gas and fuel to "pre-charge preheat" the scrap moving on a conveyor in a special horizontal preheater tunnel. The scrap is fed into the furnace through the hole in the shell side wall. The off-gas flows counter-current to the scrap. The EAF maintains a liquid heel following tapping. Electric energy consumption in the range 350-400 kWh/ton is too high, when compared to current electric arc furnace consumption standards. The apparatus by itself is requires a large space for conveyors. Scrap preheating on conveyors is not very energy efficient, because scrap is preheated predominantly from above.
U.S. Pat. No. 4,852,858 (01.08.1989) describes a "Charging Material Preheater for preheating charging materials for a Metallurgical Smelting Unit". This process known in the industry as "Energy Optimising Furnace" has favourable results and is used in production. However, this semi-continuous vertical scrap preheating apparatus with controlled scrap descent, is an integral part of a non-electric metallurgical furnace using counter-flow hot waste gases from almost complete post combustion in the furnace vessel located under the preheating apparatus. Favourable operating results of this design eventually induced some designers of electro-metallurgical equipment to adapt this, considerably modified, concept to the electric arc furnace. Overall height and large, uncontrolled infiltration of the false air into individual chambers are considered as drawbacks.
U.S. Pat. No. 5,153,894 "(06.10.1992) describes a smelting plant with removable shaft-like charging material preheater", known in the industry as a "Shaft Furnace", is a batch charged, smelting plant with shaft like material preheater which is an integral part of the furnace and with counter-current hot gas flow. By a horizontal relative movement between the furnace vessel and the holding structure, together with the vessel cover, charging material can be charged from a scrap basket directly into the furnace vessel or through the displaced shaft into different regions of the furnace vessel. Charging material can be retained in the shaft by means of a blocking member therein, and heated up during the refining phase. One of the alternatives has several design problems such as a complicated design manifested by batch charging into furnace from the shaft, two pivoting assemblies in order to allow direct top charging or to exchange shell, disfigured shape of the shell due to side mounted shaft structure for scrap preheating, tilting arrangement of the shell only, creates a large gap between shell and roof resulting in heat loses, uncontrolled scrap descent through the shaft causing occasional jamming and sliding of large portion of scrap, improper counter-current flow of gases through the shaft resulting in uneven preheating of the scrap in the shaft and two serious processing system problems: temporary, uncontrollable creation of explosive mixture and emission of toxic substances due to low waste gases temperature at the exit from the shaft. In addition to the fact that reheating of this gases via burners is in principle and de facto defeating the purpose of this type of preheating system, the possibility of emission of toxic substances and explosions are not eliminated and they occur from time to time on each of these kind of furnaces.
U.S. Pat. No. 5,264,020 (23.11.1993) describes a "Smelting plant with two melting furnaces arranged in juxtaposed relationship", known in the industry as "Double Shaft Furnace". This is actually an aggregate of two Shaft Furnaces in juxtaposed relationship and which are operated alternately, wherein the furnace gases which are produced in the melting process are respectively introduced into the other melting furnace for the purposes of preheating the charging material. Associated with each melting furnace is a shaft which is loaded with charging material. The waste gases from the furnace which is in the melting mode of operation are introduced from the shaft, after charging of the other furnace, through the cover of the other furnace and are removed from the shaft thereof. That procedure, throughout the entire smelting operation, permits preheating of the charging material and filtration of the furnace gases when they are passed through the charging material. Since "Double Shaft Furnace " is de facto very similar to the "Shaft Furnace" with slightly different charging arrangement, all of the comments concerning "Shaft Furnace" are to applicable also to this furnace aggregate.
U.S. Pat. No. 5,499,264 (12.03.1996) describes "Process and arrangement for operating a double furnace installation", known in the industry as "Twin-shell furnace". This arrangement is also de facto an aggregate of two practically complete mechanical assemblies of single electric arc furnaces, eventually in juxtaposed relation. It is disclosed as: a process for operating a double furnace installation having two arc furnaces connected via a line, a power supply, a device for charging material, and an arrangement for extraction and purification of gas. The process including the step of connecting a first one of the two furnaces with the power supply for melting a charge located therein, completely cutting off a second one of the furnaces from the power supply. The second furnace is the charged with charging material and is closed with a cover. Flue gas located in the closed second furnace is sucked out above the burden column and flue gas is sucked out of the first furnace above the surface of the melted charge through the second furnace via the connection line provided between the two furnaces. A flue gas connection of the first furnace to the gas purification arrangement is interrupted while the flue gas is being sucked out of the second furnace while feed air is simultaneously taken on in the region of a cover of the first furnace.
In principle, the flow of preheating gases is counter current with respect to the scrap. Higher productivity is achieved with complex design of exhaust system. Scrap preheating is non-uniform, resulting in higher oxidation losses. Top charging of the two vessels still requires removing of the furnace roof, resulting in additional heat losses.
U.S. Pat. No. 5,513,206 (30.04.1996) describes an "Apparatus for preheating and charging scrap material". This apparatus for preheating and charging scrap material encompasses a shaft like preheating chamber and charging unit. The furnace exhaust gas flows counter current to the falling scrap. A two stage scrap pusher delivers the charge through the opening in the roof into a space between the two DC electrodes. The two electrode DC furnace receiving preheated scrap is completely sealed and does not use water-cooled wall panels. This furnace is of extremely complex design. Scrap pushing is complicated. The shaft is narrow and therefore is equipped with several anti-bridging devices. Also the high overall height is a significant drawback.
U.S. Pat. No. 5,555,259 (10.09.1996) describes a "Process and device for melting down scrap", known in the industry as "Contiarc". The disclosed furnace is a DC arc-heated shaft furnace with an annular shaft formed by outer and inner vessels that surround and protect a central graphite electrode. Scrap is fed continuously with an appropriate system in the upper part of the annular shaft at a rate corresponding to the melt-down rate in the lower section of the furnace. During its descent, scrap is preheated by the ascending gases. When these gases leave at low temperature from the top of the scrap column, they are captured in a ring duct and conveyed away for waste gas treatment. It is claimed that this furnace will have low volume of dust emission through off-gases owing to the filtering effect of the scrap column. This design is in accordance with efforts to combine an electric furnace and scrap preheating in one, amalgamated design. Scrap charging system is complicated, scrap descent is not controlled, therefore bridging will occur. Furnace has not tilting mechanism and therefore replacement or exchange of the bottom part will be difficult.
U.S. Pat. No. 5,573,573 (12.11.1996) describes an "Electric arc furnace arrangement for producing steel", known in the industry as "Comelt". Disclosed is an electric arc furnace for the production of steel by melting scrap, in particular iron scrap, and/or sponge iron and/pig iron as well as fluxes in a furnace vessel, into which at least one graphite electrode projects, which is displaceable in its longitudinal direction, wherein an electric arc is ignited between the graphite electrode and the charging stock. To achieve particularly high energy input, the sloping graphite electrode projects into a lower part of the furnace vessel from a side and the lower part, in the region of the graphite electrode, has an enlargement radially protruding outwardly relative to the upper part. The furnace has an extended vertical shaft and it is continuously charged with cold scrap via a conveyor. It is claimed that off-gases, in counter flow to descending scrap are at the top of the shaft, after preheating the scrap, still sufficiently hot and rapidly cooled by dilution, so that no toxic gases are evolved. In another version, gases are collected. This is a complex, amalgamated design claiming very low electric energy consumption.
In addition, all of the above prior art methods have a fundamental, indubitable disadvantage and drawback: Counter-current flow of hot waste gases to the flow of the scrap is a fundamental, functionally adverse, feature. In the majority of cases prior art scrap preheating apparatus, devices and process systems this is the main reason for their unsatisfactory performance.
In summary, with respect to productivity, energy saving, pollution and safety of operation, the results from efforts aimed at intensification of electric arc furnace steelmaking processing predominantly from scrap, using Prior Art scrap preheating equipment, clearly indicate that their level of achievement is below achievable performance levels for a "pre-charge" scrap preheating apparatus properly applying fundamental laws of physics and correct exploitation of practical experience.